Motivation

On-chip optical sensors are compact platforms for sensitive, low-cost monitoring of biological and chemical species in real time by measuring changes in light that interacts with the analyte on a chip. Mid-infrared (MIR) sensors could play a vital role in biological and chemical sensing because many relevant molecules exhibit strong vibrational resonance in the MIR spectrum of λ~3-14 µm. Quantum cascade lasers (QCL) are a common source of MIR. As QCL is a very expensive platform to build the sensor (with source, interaction medium, and detector), there is a greater need to build the sensors with discrete material source, interaction medium and detector. To realize such a scheme a high-efficiency, low-cost inter-chip optical coupling technique is required.

Solution

Optical “Quilt Packaging®” (OQP) is a novel microelectromechanical system (MEMS) based packaging technique for low-cost, highly efficient optical coupling between MIR laser sources and interaction platforms. Waveguides of separate substrates are aligned with sub-micron accuracy by protruding, lithographically defined interdigitated copper nodules placed on the side of the chip. Chip-to-chip optical integration via Optical Quilt Packaging allows for direct optical interconnects between semiconductor optical sources, on-chip beam-combining optics, optical waveguides, and detectors.
In Optical Quilt Packaging waveguides of separate substrates are aligned with sub-micron accuracy by protruding, lithographically-defined interdigitated copper nodules on the side of the chip. The technique inherently provides extremely wide spectral coverage with sources ranging from the visible, infrared, and mid-infrared. At the same time it presents the scope to design separately optimized optical sources, interaction mediums, and detectors preferred for enhanced performance on chip mid-infrared optical sensing. It is possible to combine optical waveguide chips made with both Si/Ge and III-IV materials into a single heterogeneous platform using OQP using conventional fabrication processes.

We fabricated initial test OQP structures on a bare Si wafer. The double-trench ridge waveguide structures on two separate chips were aligned via Cu nodules with a resulting misalignment of ~1 μm and an inter-chip waveguide-to-waveguide distance of ~10 μm (left figure below). We are working on fabricating this OQP structure on a Ge-on-Si platform, which could be used as the sensing platform of the modular mid-IR detection system (right figure below).